In this case, when the user tried to render after enabling the OptiX denoiser, they got this error:
ERROR | [gpu] OptiX version 0.30.91 is lower than the minimum required version 5.0.0
This was with MtoA 3.0.1, which means Arnold 22.214.171.124
So, that means there is an incompatible version version of the OptiX library on the system, and it’s being loaded instead of the OptiX that ships with MtoA.
I probably could have just checked the PATH setting, but I used Process Monitor to absolutely sure, and I found this:
The problem is that another renderer added itself to the PATH. That’s a bad thing 😉
The solution? Remove that folder from PATH. Create a batch file or wrapper script to set the required environment when you start Maya to use the other software. Rather like the mtoa module file sets PATH when you start Maya.
NOTE In previous versions of MtoA, this would prevent MtoA from even loading.